1. Field of the Invention
The present invention relates to a planar light source for illuminating a liquid crystal panel and the like and a method of manufacture of the same.
2. Description of the Prior Art
Small liquid crystal displays have been used in recent years in cellular phones and other portable information terminals and, as a means for illuminating such liquid crystal displays, a planar light source is used. The planar light source is constructed of a plate-like light guide plate and light emitting diodes (LEDs) arranged to face a side surface of the light guide plate. Thanks to its ability to be reduced easily in size and thickness, the planar light source has found wide uses.
In the planar light source, light emitted from the LEDs enters into the light guide plate and propagates as it is repetitively reflected within the light guide plate. The light is reflected or refracted by grooves or a textured pattern formed in an underside of the light guide plate until it goes out of the plate. The light emitted from the top surface of the light guide plate travels toward and illuminates the liquid crystal display.
An example of such a conventional planar light source is shown in FIG. 4 (e.g., Japanese Patent Application No. 2002-093383, FIG. 7). FIG. 4 illustrates a construction of a planar light source 110, FIG. 4A and FIG. 4B representing a perspective view and a side view thereof respectively.
In FIG. 4, the planar light source 110 has LEDs 101 as a light source, a light guide plate 102, a prism sheet 103, a reflector plate 106 and a liquid crystal panel 107. The light guide plate 102 is rectangular and made of a light-transmitting glass or resin. Denoted 102a is a top surface of the light guide plate 102. Designated 102c is a light receiving side surface facing the LEDs 101. Designated 102b is a bottom surface of the light guide plate 102. The bottom surface 102b is formed with a plurality of asymmetric prisms 102b1 facing the top surface 102a. The asymmetric prisms 102b1 each comprise a down slope 102b11 whose distance to the top surface 102a sharply increases as it moves away from the light receiving side surface 102c and an up slope 102b12 whose distance to the top surface 102a moderately decreases. Arranged opposite the light receiving side surface 102c are three LEDs 101 supported on a retainer member 101b. 
When a predetermined amount of electricity is supplied from a power supply not shown to the LEDs 101, the LEDs 101 illuminate in white or a predetermined color. The light emitted from the LEDs 101 is refracted by the light receiving side surface 102c as it enters the light guide plate 102. The light that has entered the light guide plate 102 is repetitively reflected between the top surface 102a and the bottom surface 102b of the light guide plate 102 before it is refracted by the top surface 102a and leaves the light guide plate 102. The light then enters the prism sheet 103, in which it is specular-reflected until its propagation direction is changed to a Z direction. The light traveling in the Z direction is now incident on the liquid crystal panel 107. Therefore, the light passes through the liquid crystal in an ideal direction, making a clear and vivid image display possible.
FIG. 5 is a side view showing a path of light emitted from the LEDs 101 that has entered the light guide plate 102. In the figure, a light ray emitted from the LEDs 101 at an output or emittance angle of θi enters the light receiving side surface 102c of the light guide plate 102 at an incidence angle of θi. At this time, the ray is refracted on this plane and a relationship between the incidence angle θi and a refracted angle θ is, according to Snell's law, n·sinθ=sinθi assuming that a refractive index of air is 1 and a refractive index of the light guide plate 102 (made of polycarbonate and the like) is n. From this we obtainθ=sin−1((1/n)sin θi)  (1)If, for example, the light guide plate 102 has a refractive index of n=1.58 and θi=90°, calculating the equation (1) results in θ=sin−1(1/1.58)=39.3° and thus the critical angle θc is θc=39.3°.
It should be noted, however, that since the incidence angle in reality is less than 90° at maximum, the refracted angle θ even at its maximum is less than the critical angle θc. The critical angle θc of the light guide plate 102 is generally around 40°, so the refracted angle θ even at its maximum does not exceed 40°. The light ray that has passed through the light receiving side surface 102c at the refracted angle θ is incident on the top surface 102a of the light guide plate 102 at an incidence angle θ1. At this time, as can be seen from FIG. 5, since a relation of (θ+θ1=90°) holds and the refracted angle θ is equal to or less than 40°, as described above, the incidence angle θ1 is equal to or more than 50°, which is larger than the critical angle θc of around 40°. Thus, the ray incident on the top surface 102a is totally reflected at a reflection angle θ1.
The reflected light then strikes, at an incidence angle of θ2=θ1−α, the up slope 102b12 of the bottom surface which has an inclination angle of α. Here the inclination angle α is about 1° to several degrees.
The ray that has struck the up slope 102b12 at an incidence angle θ2 is reflected by this surface at a reflection angle θ2 and then strikes the top surface 102a at an incidence angle of θ3=θ2−α=θ1−2α. The ray is then reflected by the top surface 102a at a reflection angle θ3to hit the up slope 102b12 at an incidence angle of θ4=θ3−α=θ1−3α. Each time the light ray, that was first reflected by the top surface 102a at a reflection angle θ1, strikes the up slope 102b12 or the top surface 102a, its incidence angle decreases by an amount equal to the inclination angle α. That is, when the ray, that was first reflected at a reflection angle of θ1, strikes the up slope 102b12 or top surface 102a for an Nth time after repetitive reflections, its incidence angle θN is given byθN=θ1−(N−1)α  (2)In this light guide plate, the light incidence or reflection on its boundary surface, shown at θ1, is counted as the first incidence/reflection (i.e., N=1).
When the decreasing incidence angle θN has the following relation with the critical angle θc:θN=θ1−(N−1)α<θc  (3)then, the ray passes through the top surface 102a or the up slope 102b12 of the bottom surface 102b and gets out of the light guide plate 102. For example, if θ1=52°, α=1° and θc=40°, the condition of equation (3) is met when N is more than 13. This means that the light ray must strike the top or bottom surface of the light guide plate 102 fourteen times or more. Therefore, near the light receiving side surface 102c the ray does not escape to the outside. For example, if the light guide plate 102 has a thickness of 1 mm, the ray does not normally exit the light guide plate 102 from within about 3 mm of the light receiving side surface 102c. As a result, of the area of the light guide plate 102 the region that can be used as a light generation region decreases, reducing a space efficiency, which is detrimental to a size reduction of the device.
Thus, the planar light source 110 of FIG. 4, rather than being used as is, is often improved as shown in FIG. 6. FIG. 6A is an overall side view and FIG. 6B is an enlarged view of a portion C of FIG. 6A. In FIG. 6, denoted 120 is an improved planar light source in which a reference number 102bh represents a textured reflection surface provided on the bottom surface 102b of the light guide plate 102 near the light receiving side surface 102c. The textured reflection surface 102bh has an irregular pattern of fine, recessed and raised portions. The planar light source 120 therefore has on the underside of the housing 102 a reflection means provided by the textured reflection surface 102bh in addition to a reflection means provided by the asymmetric prisms 102b1. Designated 102D is a step formed at a boundary between the reflection means of the asymmetric prisms 102b1 and the reflection means of the textured reflection surface 102bh. In other respects, the symbols and construction are similar to those of the planar light source 110 shown in FIG. 4. The step 102D either rises or falls, and its edge has an angle of nearly 90°.
As shown in FIG. 6, of the light rays that have entered from the LEDs 101 through the light receiving side surface 102c into the light guide plate 102, some rays strike, and are scattered by, the textured reflection surface 102bh before traveling directly toward the top surface 102a and some rays go out of the light guide plate 102 and are reflected by the reflector plate 106 to reenter the light guide plate 102 and travel toward the top surface 102a, as indicated by solid lines in FIG. 6A. So, there are light paths involving the textured reflection surface 102bh in addition to the light paths using the asymmetric prisms 102b1, as indicated by a dashed line. As a result, light can be emitted upward from even an area of the top surface 102a of the light guide plate 102 which is close to the light receiving side surface 102c, thus expanding the illumination area to near the light receiving side surface 2c. 
However, even the improved light guide plate 102 often experiences the following problems. As shown in a plan view of FIG. 7, in an area S1 within 3–4 mm of the light receiving side surface 102c of the light guide plate 102 several bright lines 14 show up (in FIG. 7 the bright lines are shown hatched with thick lines). S2 represents an area where bright lines do not show. The conspicuous bright lines 14 are considered to be caused as follows. As shown in FIG. 6B, light rays from the LEDs 101 that entered the light receiving side surface 102c reach an edge portion of the step 102D in the bottom surface 102b of the light guide plate 102.
If the edge portion has a rough surface, rather than a mirror surface, the light rays from the LEDs 101 enter the edge portion not through normal refraction but through scattering. That is, from the edge portion, a plurality of rays travel through the light guide plate 102 in different directions, making the edge portion look as if it were illuminating. Thus, the edge portion can be regarded as a secondary light source. Since the edge portion is formed at right angles, its transferability in a molding process is bad, rendering its surface rough, which in turn results in a secondary light source being easily formed.
Next, as shown in FIG. 8 the secondary light source at the step 102D emits rays of light in various directions. Of these rays, those that are incident on the top surface 102a of the light guide plate 102 at incidence angles less than the critical angle θc pass through this surface by refraction and get out of the light guide plate 102, as indicated by rays s21, s22. This direct outward transmission of these rays occurs continuously in a wide range of area and therefore no bright lines are produced. When the incidence angles exceed the critical angle θc, the light rays are reflected by the top surface 102a and directed toward the bottom surface 102b, as indicated by rays S31, S32, S33. Then, after one to several reflections these rays pass through the top surface 102a and go out as illuminating light. The number of times that these rays are reflected before being emitted outside increases as the first incidence angle on the top surface 102a becomes larger, according to the principle already explained (see Equation (3)).
That is, the number of reflections increases, from 1 to 2 to 3, according to the light paths S31, S32, S33. Hence, the positions on the top surface 102a of the light guide plate 102 from which the light rays go out are separated from each other, resulting in discrete bright lines as shown in FIG. 7. As for a width of light flux, let us turn to FIG. 9 and compare light fluxes φ1 and φ3 which correspond to the light paths s31 and s33 of FIG. 8. Let a width of light flux φ1 as it exits the top surface 102a of the light guide plate 102 be b1 and a width of light flux φ3 be b3. It is seen that the light width b3 is significantly larger than the width b1. This is considered due to the fact that as the number of reflections for each light path increases, a length of the light path also increases and, almost in proportion to the path length, the width of the light flux increases. Thus, as shown in FIG. 7, the width of the bright lines progressively widens away from the light receiving side surface 102c of the light guide plate 102. As the number of reflections for each light path increases further and the width of each flux exiting the top surface 102a of the light guide plate 102 widens, a light quantity per unit area, i.e., brightness lowers, with the result that the bright lines become indistinguishable in an area more than a certain distance away from the light receiving side surface 102c, as shown in FIG. 7.
As described above, the marked bright lines are caused by the step 102D (see FIG. 6 and FIG. 8) in the bottom surface 102b of the light guide plate 102. This step 102D is formed by a plurality of inserts (in-cavity molding pieces having recessed and raised transfer surfaces) during the process of molding the light guide plate 102. FIG. 10 illustrates essential parts of a conventional mold used in molding the light guide plate 102. In FIG. 10, denoted 121 is a mold frame, 122 an insert for textured pattern, and 123 an insert for prism pattern. The insert 122 for the textured pattern has its surface formed with a pattern of undulations corresponding to the textured reflection surface 102bh of FIG. 6, and the insert 123 for the prism pattern has its surface formed with a pattern of recesses and projections corresponding to the asymmetric prisms 102b1 provided on the bottom surface 102b of the light guide plate 102. The light guide plate 102 is formed in the following process. First, as shown in FIG. 10A, the insert 122 for the textured pattern and the insert 123 for the prism pattern are placed inside the mold frame 121. Next, as shown in FIG. 10B, both of these inserts 122, 123 are set close together. Then, a melted resin is injected into the mold frame 121 to mold the light guide plate 102 and transfer the surface patterns of these inserts 122, 123 to the light guide plate 102.
In this process, as shown in a cross section of FIG. 10C, a step D is often formed between the textured pattern insert 122 and the prism pattern insert 123 and is transferred onto the light guide plate 102 as the step 102D of FIG. 6. The edge of the step D is almost at right angles. It is noted that the step D, which is formed by a difference in thickness between the textured pattern insert 122 and the prism pattern insert 123, is very difficult to eliminate by equalizing the thickness of these inserts because the thicknesses of the inserts 122, 123 change when forming their recessed/raised surface patterns.
As described above, the step D formed between the inserts 122 and 123 is transferred to the conventional light guide plate 102 as the step 102D in the reflection surface. So, when light is illuminated from the light guide plate 102 of the above construction, bright lines produced by this step show up. The bright lines in turn form bright and dark fringes, marring the appearance of the planar light source.